1. Field of Invention
The invention relates to a hydraulic control system for an automatic transmission. More specifically, it relates to the hydraulic control system, which controls an output pressure with a different amplification factor related to a signal pressure, for an automatic transmission.
2. Description of Related Art
Generally, in a conventional hydraulic control system for an automatic transmission, a linear solenoid valve outputs a signal pressure, the signal pressure is input to a primary regulator valve, and the primary regulator valve regulates a line pressure to an output pressure properly based on the signal pressure. Then, many kinds of controls are performed based on the output pressure. A change rate of the output pressure from the primary regulator valve related to a change in the signal pressure output from the linear solenoid valve is made using an amplification factor, which is called a gain from now on. The gain is set so that the line pressure is regulated to the output pressure needed for the hydraulic controls within the range of the signal pressure which is output from the linear solenoid valves. That is to say, the gain is set so that the change range of the output pressure is achieved within the change range of the signal pressure.
For example, in a continuously variable transmission which performs non-stage shift by changing a pulley ratio between two pulleys joined with a belt, an unusually high output pressure is needed. That is to say, in such a continuously variable transmission, the pulley ratio is changed by increasing and decreasing a tension on the belt by a fixed sheave and a movable sheave that comprise each pulley. The tension on the belt needs to change over a wide range. Therefore, the output pressure from the primary regulator valve for achieving the tension on the belt over the wide range needs to change over a wide range. As a way for achieve the wide change range of the output pressure, the change range of the signal pressure may be made large. In this case, the linear solenoid valve needs to be large to achieve the wide change range for the signal pressure. Therefore, a large space for positioning the large linear solenoid valve is needed and costs increase.
Another way to achieve the wide change range of the output pressure considering the aforementioned disadvantages, it is possible to make the gain large. In this case, when the gain is large, the change in the output pressure is large related to the change in the signal pressure. Then, however, the dispersion of the output pressure becomes large due to vibration in the signal pressure. Therefore, it is difficult to achieve high accuracy control for the output pressure. That is, in the case where the change range of the signal pressure is stable, when the gain is set to be large, the change range of the output pressure becomes large and the high output pressure is achieved, but high accuracy control for the output pressure is difficult. When the gain is set to be small, high accuracy control for the output pressure is achieved, but the change range of the output pressure becomes small and the necessary high output pressure is not obtained.
Actually, in a vehicle mounting the aforementioned continuously variable transmission, the continuously variable transmission needs the large output pressure at, for example, the vehicle start because the pulley ratio must be greatly changed. And, for example, during a settled speed driving, the continuously variable transmission needs a small output pressure for the purpose of, for example, minimizing fuel consumption and maintaining a high accuracy control for the output pressure. That is to say, in the continuously variable transmission, when the continuously variable transmission needs low output, a small gain is proper. When the continuously variable transmission needs high output, a large gain is proper.
The linear solenoid valves and regulator valves have dispersions caused by, for example, manufacturing accuracy and assembly accuracy of the valve body and spools, and the spring load of the spring loaded spools in each assembly. Therefore, the signal pressure from the linear solenoid valves and control pressures from the regulating valves have a dispersion related to the same current value in each hydraulic circuit. That is, the control pressure as output, related to the current value as input, has a degree of dispersion.
For reducing the dispersion in each hydraulic circuit, an adjusting mechanism is arranged in the linear solenoid valve. Used as the adjusting mechanism, for example, is an adjusting screw for increasing and decreasing the spring load of the spring loaded spool. In this case, the spring load can be increased and decreased by the adjusting screw in the each hydraulic circuit and, then, the signal pressure and the control pressure related to the signal pressure are regulated. The proper control pressure is thereby achieved.
FIG. 12 shows an example for regulating the control pressure in the related art. FIG. 12 shows linearly changing the control pressure P.sub.L, with the dispersion found in various hydraulic circuits related to the current values I of the linear solenoid valves. In FIG. 12, solid line A shows a target value, upper side alternating long and two short dash line a.sub.1 shows a maximum value of the dispersion before adjustment, a lower side alternate long and two short dash line a.sub.2 shows a minimum value of the dispersion before adjustment, an upper side alternate long and short dash line b.sub.1 shows a maximum value of the dispersion after adjustment, and a lower side alternate long and short dash line b.sub.2 shows a minimum value of the dispersion after adjustment.
The control pressure P.sub.L is adjusted as follows.
In the related art, an adjusting point (basic point) is set on a low pressure side of the control pressure P.sub.L, and the control pressure P.sub.L is regulated so that the control pressure P.sub.L is a predetermined control pressure P.sub.A. Actually, the current value of the linear solenoid is maintained at a predetermined value. Then, while watching the control pressure output from the regulator valve, a thrust amount of the adjusting screw is adjusted, and the adjustment is ended when the control pressure P.sub.L reaches the predetermined control pressure P.sub.A.
When adjusting from the upper side alternate long and two short dashes line a.sub.1, the thrust amount of the adjusting screw is increased and the left end of the line a.sub.1 is set to the predetermined control pressure P.sub.A. When adjusting from the lower side alternate long and two short dashes line a.sub.2, the thrust amount of the adjusting screw is decreased and the left end of the line a.sub.2 is set to the predetermined control pressure P.sub.A. By this process, the lines a.sub.1, a.sub.2 are displaced in parallel and changed to the lines b.sub.1, b.sub.2.
By performing the adjustment for the each hydraulic circuit, the predetermined control pressure P.sub.A is achieved for the predetermined current value of the linear solenoid valve in each hydraulic circuit. That is, the predetermined control pressure is set to correspond to the predetermined current value.
But in the related art, the dispersion after adjustment is large when the control pressure P.sub.L is high because the adjusting point is set for the low pressure side of the control pressure P.sub.L. Therefore, the hydraulic pressure that may be output is too great compared with the target value A shown in FIG. 12. As a result, it is necessary to consider the strength and durability of the hydraulic circuit when such a high pressure results.
Conversely, when the adjusting point is set at the high pressure side of the control pressure P.sub.L, the dispersion is reduced at the adjusting point on the high pressure side, but the dispersion is increased at the low pressure side. Therefore, it is necessary to set the lowest control pressure high so that the control pressure is enough for the large dispersion at the low pressure side.
When the adjusting point of the control pressure P.sub.L is set on the low pressure side, the dispersion is increased at the high pressure side, and when the adjusting point of the control pressure P.sub.L is set on the high pressure side, the dispersion is increased at the low pressure side. That is to say, the dispersion of the control pressure P.sub.L is increased in a side which is far away from the adjusting point because the adjustment by the adjusting screw does not change the slope of the lines in FIG. 12, it just displaces them in parallel.